GEC Code (Marconi/Osram Brands also):

Consist of Letter (or Letters) followed by digits (e.g. L63, KT88). The digits are simply to distinguish similar valves (like all triodes), and cannot be decoded. The letters have the following meaning.

A - Industrial valve (Maybe almost anything - power triode, etc)
B - Double triode
D - Diode
GU - Gas-filled rectifier
GT - Gas Triode (Thyratron)
H - signal triode (high impedance)
KT - Kinkless tetrode (beam tetrode)
L - signal triode (low impendance)
MU - indirectly heated rectifier
N - output pentode
P - output triode
QP - Quiescent push-pull double pentode
S - Tetrode (Screen Grid Valve)
U - rectifier
VS - variable mu tetrode
W - variable mu pentode
X - triode hexode, heptode, octode, frequency changer
Y - tuning indicator
Z - HF pentode